The
following guidelines are presented for those machinists not familiar with
post-machining annealing (stress-relief) for high-performance plastics.
They are intended as guidelines only, and may not represent the most
optimum conditions for all parts.

Most
quality stock shape materials are stress-relieved by the manufacturer after
molding to ensure the highest degree of machinability
and dimensional stability. However, some components may benefit from
post-machining annealing or stress-relieving using the instructions below.

Post-Machining
Annealing (Stress-Relief)

When should parts be annealed after
machining to ensure optimum part performance?
Experience has shown us that very few machined plastic parts require
annealing after machining to meet dimensional or performance requirements.

Generally
speaking, most stock shapes are annealed after molding using a proprietary
stress relieving cycle to minimize any internal stresses that may result
from the manufacturing process. This assures you that the material will
remain dimensionally stable during and after machining.

Machined-in
stress can reduce part performance and lead to premature part failure. To
prevent machined-in stress, it is important to identify the causes.
Machined-in stress is created by:

Using dull or improperly
designed tooling

Excessive heat
–– generated from inappropriate speeds and feed rates

Machining away large
volumes of material –– usually from one side of the stock
shape

To
reduce the potential for machined-in stress, review the fabrication
guidelines for the specific material. Recognize that guidelines change as
the material type changes.

BENEFITS OF POST-MACHINING ANNEALING

Improved Chemical
Resistance
Polycarbonate, polysulfone, and Ultem® PEI, like many amorphous (transparent)
plastics may be annealed to minimize stress crazing. Torlon® PAI also benefits from post machining
annealing. Annealing finished parts becomes more important as
machining volume increases. Annealing after machining reduces
"machined-in" stresses that can contribute to premature
failure.

Better Flatness &
Tighter Tolerance Capability Extremely close-tolerance parts requiring
precision flatness and non-symmetrical contour sometimes require
intermediate annealing between machining operation. Improved flatness
can be attained by rough machining, annealing and finish machining
with a very light cut. Balanced machining on both sides of the shape
centerline can also help prevent warpage.

Improved Wear Resistance Extruded or injection
molded Torlon® PAI parts that require
high PV's or the lowest possible wear factor benefit from an
additional cure after machining. This curing process optimizes the
wear properties. Only PAI benefits from such a cycle.

POST MACHINING AIR ANNEALING GUIDELINES

Material

Heat Up

Hold

Cool Down

Environment

ABS

50°F per hour
to 200°F

30 minutes per
1/4” thickness

50°F per hour

Nitrogen

Acrylic

2 hours to
180°F

30 minutes per
1/4” thickness

50°F per hour

Nitrogen

Acetal copolymer

4 hours to
310°F

30 minutes per
1/4” thickness

50°F per hour

Nitrogen
or Air

Delrin® acetalhomopolymer

4 hours to
320°F

30 minutes per
1/4” thickness

50°F per hour

Nitrogen
or Air

Ardel® polyarylate

50°F per hour
to 330°F

30 minutes per
1/4” thickness

50°F per hour

Nitrogen

Ertalyte® PET-P

4 hours to
350°F

30 minutes per
1/4” thickness

50°F per hour

Oil or Nitrogen

Halar® ECTFE

50°F per hour
to 225°F

30 minutes per
1/4” thickness

50°F per hour

Nitrogen

Hydex® 4101 PBT-P

4 hours to
300°F

60 minutes per
1/4” thickness

50°F per hour

Nitrogen
or Air

Kynar® PVDF

2 hours to
275°F

30 minutes per
1/4” thickness

50°F per hour

Nitrogen

Noryl® PPO

50°F per hour
to 250°F

30 minutes per
1/4” thickness

50°F per hour

Nitrogen

Noryl® PPO
30% glass filled

50°F per hour
to 260°F

30 minutes per
1/4” thickness

50°F per hour

Nitrogen

Nylon - type 6

4 hours to
300°F

30 minutes per
1/4” thickness

50°F per hour

Oil or Nitrogen

Nylon - type 6/6

4 hours to
350°F

30 minutes per
1/4” thickness

50°F per hour

Oil or Nitrogen

Nylon - glass-filled

4 hours to
375°F

30 minutes per
1/4” thickness

50°F per hour

Oil or Nitrogen

PCTFE (formerly Kel-F®)

3 hours to
225°F

60 minutes per
1/4” thickness

50°F per hour

Air

PEEK polyetheretherketone

2 hours to
300°F
then
2 hours to 375°F

60 minutes per
1/4” thickness

60 minutes per 1/4” thickness

50°F per hour

Air

Polycarbonate (unfilled)

4 hours to
275°F

30 minutes per
1/4” thickness

50°F per hour

Air

Polycarbonate (glass-filled)

4 hours to
290°F

30 minutes per
1/4” thickness

50°F per hour

Air

Polyethylene (UHMW)

2 hours to
220°F

30 minutes per
1/4” thickness

10°F per hour

Nitrogen

Polypropylene

2 hours to
185°F

30 minutes per
1/4” thickness

50°F per hour

Air

Polystyrene

50°F per hour
to 170°F

30 minutes per
1/4” thickness

50°F per hour

Nitrogen

Radel® R polyethersulfone

4 hours to
390°F

30 minutes per
1/4” thickness

50°F per hour

Nitrogen
or Air

Ryton® PPS

4 hours to
350°F

30 minutes per
1/4” thickness

50°F per hour

Air

Techtron® PPS

4 hours to
350°F

30 minutes per
1/4” thickness

50°F per hour

Air

Torlon® PAI

4 hours to
300°F
then
4 hours to 420°F
then
4 hours to 470°F
then
4 hours to 500°F

1 day

1 day

1 day

3 to 10 days

50°F per hour

Air

TPX® polymethylpentene

50°F per hour
to 200°F

30 minutes per
1/4” thickness

50°F per hour

Nitrogen

Udel® polysulfone

4 hours to
330°F

30 minutes per
1/4” thickness

50°F per hour

Air

Ultem® PEI (unfilled)

4 hours to
390°F

30 minutes per
1/4” thickness

50°F per hour

Air

Ultem® PEI (20%, 30% glass filled)

4 hours to
400°F

30 minutes per
1/4” thickness

50°F per hour

Air

Finish
machining of critical dimensions should be performed after annealing.

Important:
Annealing cycles have been generalized to apply to a majority of machined
parts. Changes in heat up and hold time may be possible if cross sections
are thin. Parts should be fixtured during
annealing to prevent distortion.

Trademark
Acknowledgments:

RADEL and TORLON are registered trademarks of Solvay
Engineering Polymers.
RYTON is a registered trademark of Chevron Phillips Chemical Company.
DELRIN is a registered trademark of DuPont.
ULTEM is a registered trademark of General Electric Company.
HYDEX is a registered trademark of A.L. Hyde Company.
ERTALYTE and TECHTRON are registered trademarks of Quadrant Engineering
Plastic Products.
PEEK is a trademark of Victrex plc.

The information on this page originally provided by Quadrant Engineering
Plastics & Westlake Plastics Company.
Other stock shape manufacturers have also provided relevant information.